General Description

Beta decay is a fundamental decay mode of a nucleus wherein one neutron in the
nuceus is converted to a proton, an electron (beta particle), and an anti-neutrino.
For about 60 nuclei, this decay is energetically forbidden to occur
[the product nucleus has a higher mass] while the (very rare) double-beta
decay process is allowed. Here, two neutrons are simultaneously
converted into two protons, two electrons, and two anti-neutrinos.
This rare decay mode has been observed in only 11 of the 60 possible cases. For example,
Germanium-76 (76Ge) -- which accounts for less than eight percent of naturally
occurring Germanium -- cannot decay to its neighbor, Arsenic-76, but it can decay
to Selenium-76 via double-beta decay.

An even rarer decay mode is neutrinoless double-beta decay, wherein two neutrons
convert to two protons and two electrons, but no neutrinos are emitted. For this to
happen, one neutron in the nucleus changes to a proton and emits an electron and anti-neutrino,
and then the anti-neutrino is absorbed by the second neutron, inducing it to change to
a proton and emit just a second beta particle. This can only happen if the neutrino is its
own anti-particle, or a so-called "Majorana" particle named after the theorist E. Majorana.
Never observed, it is not even known whether this decay mode exists. If it was measured, it
would demonstrate that the neutrino is a Majorana particle and provide valuable constraints
on the absolute scale of neutrino mass. Since neutrinos are fundamental particles that play
a key role in the early universe, cosmology and astrophysics, searches for neutrinoless
double-beta decay have cosmic implications.

Detailed Description

The MAJORANA Collaboration is constructing the DEMONSTRATOR, an array of germanium detectors,
to search for neutrinoless double-beta decay of germanium-76. The MAJORANA Collaboration
consists of over 100 members from 16 institutions in four countries.

Germanium detectors are a powerful tool for searches for the pair of electrons that
would be the signature for neutrinoless double-beta decay. The challenge
in such a search is the presence of background events from the low levels of natural radioactivity
that obscure any real double-beta decay events in the data stream. The excellent energy
resolution of germanium detectors (their ability to measure energy deposition very precisely)
reduces or removes many of these backgrounds.

As the detectors themselves are semiconductor devices, they are exceptionally clean,
significantly reducing potential backgrounds. There is also a long history of analyzing signals
from germanium detectors to glean specific information about the events that caused them.
This capability will help reduce backgrounds even further. The MAJORANA collaboration
plans to use devices known as "point contact detectors" in the upcoming phase of our
experiment.

Technical Description

The first phase of the MAJORANA experiment will be to deploy roughly 60 kg
of germanium detectors at the Sanford Underground Laboratory in what we call
a "Demonstrator Module." The first half of the detectors will be fabricated
from natural germanium, which is 7.44% 76Ge. We anticipate construction of
this detector array beginning very shortly after the Sanford Underground
Laboratory is ready, with production data taking to start roughly one year
later. The second half of the DEMONSTRATOR will be detectors enriched to 86%
in 76Ge. The enriched detectors will provide a much stronger double-beta decay
signal, while the unenriched detectors will allow us to begin understanding
MAJORANA backgrounds on a comparatively short timescale.

The goal of the DEMONSTRATOR is to determine whether a future 1-tonne experiment
can achieve a background goal of one count per tonne-year in a 4-keV region
of interest around the 76Ge neutrinoless double-beta decay Q-value at 2039 keV.
MAJORANA plans to collaborate with GERDA for a future tonne-scale 76Ge
neutrinoless double-beta decay search.

A CAD drawing of the cross-section of the MAJORANA DEMONSTRATOR, showing
installation of a cryostat module, is given at the right, along with one of
the individual detectors.